When drilling offshore in deep water, particularly into zones having low formation pressure from adjacent formations with higher pressure and unstable formations, there was a serious risk of collapse of the drilled hole before casing could be inserted. In the long interval between getting the drill sting out and running in casing to tie back to existing casing in the wellbore, the collapse or unstable formations in the newly drilled area would make running casing virtually impossible. The solution that was developed was to drill down just short of the pay zone and pull out of the hole with the drill sting. The casing string that was next to be secured in the well was then suspended from the floating rig. This run of casing could be almost as long as the distance from the rig floor to the seabed. The drill string would then be run through this length of casing with the lower end having an under-reamer on top of a bit extending below the suspended casing string. A mud motor was used to rotate the lower end of the drill string, while the upper end remained fixed. The fixed portion of the drill string was secured to the casing and both advanced together downhole. Drilling into the pay zone would continue until the casing got stuck due to formation collapse around it, once the pay zone was breached. Depending on the formation characteristics of the pay zone the casing could get stuck due to borehole collapse around it as soon as a few feet after drilling into the zone. At that point the casing string was secured to the next size of casing with an external casing packer. The drill string was removed and a production string and packer were run. The last run of casing was not cemented because it couldn't be due to hole collapse around it.
It is well known that offshore deepwater rigs have very high daily rates. Accordingly, when drilling particularly in very deep water it would be advantageous to save rig time where possible.
The method of the present invention allows this to occur by making it possible to drill a portion of a well and in the same trip to cement that section of casing that is run in on the stationary portion of the drill string. In the preferred embodiment a mud motor drives an under-reamer and a bit that extend below the casing supported by the drill string. A cementing/inflation tool on the drill string engages with cementing ports in the casing to pass cement outside of the casing. The casing is secured to existing casing and the drill string is removed to complete the operation. Subsequent casing runs can be secured in the same fashion. Ultimately, a production string is secured to allow production. The process can be repeated for successive casing sizes. The process is even adaptable for use on land rigs after an initial depth is drilled, cased and cemented in the traditional manner. These and other beneficial aspects of the present invention will become more apparent to one skilled in the art from a review of the description of the preferred embodiment and the claims, which appear below.